Field of the Invention
The disclosure relates generally to a system and method for measurement of the discernible properties of a fluid that relate to the composition of a fluid and the energy content of a fluid, such as steam, including a fluid attribute such as steam quality. More specifically, the disclosure relates to a system and method for measurement of the quality and related energy and composition properties of a fluid, such as steam, using electromagnetic waves.
Description of the Related Art
Fluid properties depend upon molecular composition, fluid phase and fluid internal energy state. The energy state for a given fluid phase can generally be described in terms of temperature and pressure. For example, steam energy is mainly composed of thermal energy as in heat and kinetic energy as in movement of atoms and molecules. Besides temperature and pressure as exemplary parameters for steam energy, another parameter for steam energy, which quantifies its composition with respect to phase, is steam quality. Steam quality quantifies the amount of condensed water present in saturated steam. Specifically, steam quality is defined as the mass ratio of saturated steam vapor to the total vapor plus condensate in a steam vapor- and liquid-water mixture. Although quality is an important process variable, it is not one that is easily measured in real-time. A standard method for measuring steam quality involves expensive and complicated calorimeter devices, as well as considerable time and effort. As a result, timely measurements of quality are often not available. Another standard method used in industrial settings is to measure the temperature and pressure at given points in a process flow and correlate those measurements to a database to predict the steam quality without actually measuring the quality. In the first method, the ability and time may not be available, such as in many industrial processes. In the second method that may be used for industrial processes, the accuracy may be insufficient. For example, steam turbines are operated typically at less than optimum efficiency to allow a margin of error for inaccuracies in steam quality and avoid large-scale damage. An example of such operation is described in U.S. Pat. No. 8,433,526 B2, entitled “Method and System for Steam Quality Monitoring”, where the Abstract states:                A method of determining a steam quality of a wet steam located in an interior of a steam turbine includes emitting from an optical probe a plurality of wavelengths through the wet steam, measuring with the optical probe a wet steam intensity corresponding to each of the plurality of wavelengths emitted through the wet steam, determining an intensity ratio vector by dividing the wet steam intensity by a corresponding dry steam intensity for each of the plurality of wavelengths, successively applying scaling factors to the intensity ratio vector to obtain a scaled intensity ratio vector, calculating a suitable value for each of the scaling factors to obtain a plurality of residuals, determining a minimum residual of the plurality of residuals, determining a droplet size distribution by calculating the droplet number density corresponding to the minimum residual, and determining the steam quality based on the droplet size distribution.        
It is well known that electromagnetic (“EM”) energy can be applied to materials and the resulting response comprising energy passing through or reflected from the material can be measured to indicate properties of the materials. Electromagnetic properties of materials are frequency dependent. Information about the composition of the substance can be obtained by exposing the substance to EM energy at different frequencies and analyzing the response as a function of frequency. The term “permittivity” describes how a material responds to an applied electric field. Permittivity is determined by the ability of a material to polarize in response to an externally applied field so as to reduce the total electric field inside the material. Permittivity is often expressed as a relative value which is the ratio of the complex permittivity to the permittivity ε0 of a vacuum. The response of natural materials to external EM fields depends on the frequency of the field, because the material's polarization does not respond instantaneously to an applied field. Permittivity for materials is expressed as a complex function to allow specification of the energy storage property and energy dissipation property of the material as a function of the angular frequency (ω) of the applied field by means of real and imaginary components, respectively, as follows:εr(ω)=εr′(ω)−jεr″(ω)
Magnetic permeability, as another form of a material's response to applied EM energy, can be compared with electrical permittivity in that it is the degree of magnetization of material from reordered magnetic dipoles in the material when responding to a magnetic field applied to the material. Magnetic permeability is often expressed as a relative permeability to permeability μ0 in a vacuum. Magnetic permeability is also frequency dependent and can include real and imaginary components.
The dielectric properties of water have been measured and vary considerably based on the thermodynamic state of the water. At 100° C., the real part of the static relative permittivity of water changes from approximately 56 for the liquid phase to approximately 1.012 as a gas. However, the typical range in the permittivity values seen for a mixture of water and steam is much less than the large difference in the permittivities of liquid and vapor. This reduction in sensitivity is due to the fact that the permittivity of a mixture of molecules depends upon the relative volume occupied by each constituent. As a result, the permittivity of steam varies only from about 1.012 for dry steam to about 1.08 for 50% (x=0.5) steam quality at 100° C. and 175 psia. As temperature increases, the permittivities of both dry steam and water decrease. Increasing pressure typically increases the permittivity value.
There exists a significant need for a low cost, accurate, and robust sensing method and corresponding commercially viable instrument to measure fluid properties, including the energy of steam and related steam quality and other fluid parameters such as composition, using EM energy in a number of important energy production and industrial processing applications.